In a study of nearly 500 marijuana smokers who tried to kick the habit, about one-third resumed use to relieve or avoid withdrawal symptoms such as irritability and anxiety.

There's long been a debate over whether pot smokers actually become addicted to the drug and whether withdrawal symptoms are real. They do and they are, says researcher David Gorelick, MD, PhD, of the National Institute on Drug Abuse in Baltimore.

He predicts cannabis withdrawal syndrome will be recognized as a psychiatric disorder in the next edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM), considered the bible of mental disorders. It's due out in 2012.

Gorelick presented the findings at the American Psychiatric Association annual meeting.

Heavy Pot Smokers and Withdrawal Symptoms

The study involved 469 pot smokers, ages 18 to 64, who were recruited using word of mouth and advertisements. None of the participants suffered from recognized psychiatric disorders.

About one in four reported smoking pot more than 10,000 times in their lives -- the equivalent of daily use for 27 years. More than half smoked more than 2,000 times.

"These were heavy users," Gorelick says.

A total of 42.4% experienced at least one withdrawal symptom -- most commonly, cravings, irritability, boredom, anxiety, and sleep disturbances -- when they tried to quit.

Of those who reported withdrawal symptoms, 78.4% said they started smoking pot again to reduce them.

Overall, 33.3% of participants resumed cannabis use to reduce or avoid withdrawal symptoms.

"Heavy pot users should be aware that they may experience a withdrawal syndrome that will make them uncomfortable when they try to quit," Gorelick tells WebMD.

The problem, says the University of Pennsylvania's Kyle Kampman, MD, is that doctors don't have much to offer pot users to relieve their symptoms.

Kampman is involved in a study testing oral delta-tetrahydrocannabinol, the main active ingredient in marijuana, as a potential treatment for marijuana withdrawal.

But other than to try to get patients enrolled in the trial, "the only other thing I can offer is inpatient care. Sometimes just keeping them away from marijuana will help prevent relapse," he tells WebMD.

Kampman says there is no doubt that cannabis withdrawal "is a real syndrome."